N-Protected amino acids, in which the amino groups are protected by protecting groups such as carbobenzyloxy group (hereafter referred to as Cbz group) are commonly used as materials for peptide synthesis. Among the N-protected amino acids, N-carbobenzyloxy-L-cyclohexylglycine (hereafter referred to as Cbz-CHG) is an important intermediate for certain medical drugs that contain peptide structures within their molecules, such as anti-virus drugs having serine-protease inhibitor activity and anti-arthritis drugs having metalloproteinase inhibitor activity.
A known method of producing an N-protected amino acid, Cbz-glycine in the present example, is as follows: glycine is reacted with carbobenzyloxy chloride (hereafter referred to as Cbz-Cl) under an alkaline condition (NaOH) to generate Cbz-glycine, the reaction solution is washed with a poor solvent (ether), and the crystals are then allowed to form under a cold and acidic (hydrochloric acid) condition, isolated by filtration, and recrystallized from chloroform to give Cbz-glycine (Non-Patent Document 1).
However, the method of Non-Patent Document 1 is not readily applicable to the production of Cbz-CHG, because when the reaction solution (in which Cbz-CHG has been generated under alkaline condition) is washed with a poor solvent and an acid is added, a gum-like mass instead of crystal will form.
Another commonly-used method for producing a protected amino acid is as follows: after an amino acid starting material is subjected to a reaction to generate its protected form, the product is once extracted from the reaction solution with an organic solvent that does not mix with the reaction solution, and the organic solvent is then removed by evaporation and a poor solvent is added as needed to allow the reaction product to crystallize. In the production of low-melting point amino acid derivatives, this method is known to enable crystallization of the protected amino acids of interest even from the oily materials, by the use of multiple organic solvents, a cooled organic solvent, or other variations.
The present inventors have tested the above method (hereafter also referred to as the conventional extraction/crystallization method) on Cbz-CHG, by performing organic solvent extraction of the reaction solution in which Cbz-CHG was generated under alkaline condition, and have been able to obtain Cbz-CHG crystals successfully. However, this method involves many steps such as extraction step and concentration step and therefore suffers from high cost. Another problem associated with this method is that it requires the use of large quantity of organic solvents that may be hazardous due to, for example, their toxicity or flammability.